Power transmissions, such as that shown in U.S. Pat. No. 4,070,927 issued to Polak an Jan. 31, 1978, have electro-hydraulic ratio interchange controls to establish the engagement and disengagement of the friction devices that control the drive ratio in the planetary gear set of the transmission. These control systems, such as those shown in U.S. Pat. Nos. 5,616,093 and 5,601,506 both issued to Long et al, incorporate relay valves to direct pressurized fluid to the oncoming friction device and to exhaust fluid from the off going friction device. In these control systems, two variable pressure solenoids are incorporated to control the fluid pressure in the friction devices during the ratio interchange.
The relay valves are solenoid operated mechanisms that have a spring set and a pressure set position. If the power to the solenoids is discontinued, the systems are designed to retain the fifth forward ratio if the transmission is in the fourth through sixth ratio and to retain the third forward ratio if third or lower has been established. The transmission control systems permit shifting to reverse after electrical discontinuance also. After an engine shut-down only the third, neutral and reverse conditions can be achieved. The control does not permit the establishment of the first ratio and therefore imposes some limits on the gradeability of the vehicle following an electrical discontinuance. In another control system, it is known to limit the transmission to five forward speeds such that one of the variable pressure valves can be used to provide a pressure bias for the system regulator during the fifth forward ratio.